Accidental Torsion Modelling

[Galambos]

How is the accidental torsion modelled, say, in a 3D steel frame (RISA/STAAD)? Is it more appropriate to apply a force couple, or a nodal moment? Which is most common?

 

[haynewp]

If you are inputting the force by hand, then I would just apply the force at the required accidental eccentricity distance from the centroid, then create another case where you input the force on the other side of the centroid.

If the programs are automatically generating the building mass for you (based on applied gravity loads) and applying the seismic resultant force at the centroid, I would take that resultant, multiply it times the required eccentricity, then place a moment at the centroid to represent the accidental torsion (one case for the resultant being at each side of the centroid, if you are confident in the program resultant). Actually, I would expect the programs to allow you to enter the accidental eccentricity to be considered.

I am just not sure how you made the model or the options in each program. But make sure you don't apply the force/torsion twice, one internal to the program and one input by hand.

 

[Lutfi]

In STAAD use the "OFFSET" command, then invoke the torsion desing. STAAD has a good steel torsion module.

Regards,
Lutfi